That trunked communication systems afford the most efficient use of limited frequency spectrum has become well accepted. In both the private and public markets, more and more businesses, associations, and agencies are specifying trunked systems for their communication needs. In a trunked communication system, a central controller allocates a limited number of channels among many subscribers. Generally, within a trunked system, the many subscribers are organized into fleets, sub-fleets or groups, and individuals. Accordingly, each subscriber has a fleet, group, and an individual identification (ID) code. Since the number of channels is far less than the number of subscribers, the subscribers must request and be granted a channel before they may communicate.
To initiate a group call, it is known for a requesting subscriber to transmit an information packet, commonly referred to as a "call request". The call request consists of an ID field containing the requesting subscriber's fleet, group, and individual IDs, and a digital code (referred to as a call type field), which represents the type of call desired (a group call in this example). Generally, call requests are encoded with error correcting and/or other codes to form Inbound Signalling Words (ISWs) to facilitate communication. The central controller receives the ISW and extracts the call request. If a channel is available, the central formats an information packet, commonly referred to as a "channel grant", which comprises the requesting subscriber's fleet, group, and individual IDs, and a channel assignment field, which contains a digital code representing the channel upon which communications may commence. The central encodes the channel grant into an Outbound Signalling Word (OSW), which is transmitted to all subscribers monitoring a selected one of the available channels designated as the control channel. If there is no channel available, the central formats and returns a "Busy OSW". Upon receipt of the channel grant, the requesting subscriber moves to the assigned channel and begins transmitting, while all other subscribers in the same group as the requesting subscriber move to the channel as listening units. In this way, a group call is established.
An advance in the trunked communication arts was afforded by an expanded signalling protocol described in a co-pending application by Ken Zdunek entitled "Improved Dispatch Trunked Radio System", Ser. No. 933,367, filed Nov. 19, 1986, and assigned to the same assignee as the present invention. The expanded signalling protocol employs dual ISWs to enable subscribers to request both inter-fleet and intra-fleet individual or private calls, as well as providing for advanced operational features such as Dynamic Individual Reprogramming, Dynamic Group Regrouping, Emergency Call, and other features desired or required by trunked system users. However, this advance was not without cost. In fact, it may be said that the advanced dual ISW signalling protocol contributes to control channel traffic congestion, which represents one of the potential problems of contemporary trunked communication systems. Within a single trunked system, there may be hundreds of fleets, several hundred groups, and several thousand subscribers. Given a fixed number of subscribers, the advanced dual ISW protocol automatically doubles the traffic on the control channel. Further, the congestion on the control channel may become compounded as additional subscribers are "loaded" (i.e., the central is programmed to recognize their IDs) onto the system. This may lead to unacceptable system access time in large high-use systems as subscribers are unable to place call requests on the congested control channel.
As previously mentioned, there has been, and presently continues to be, tremendous growth in both the installation of new trunked systems, and in the loading of additional subscribers onto existing systems. This continued growth, however, has identified another problem in contemporary trunked systems. That is, the problem of limited subscriber ID capacity. In present trunked systems, the ID portion of the call request is limited to sixteen (16) bits. These sixteen bits are allocated among the fleet, group, and individual IDs of a subscriber. Although it is possible, to a limited extent, to have fewer fleets and/or groups and to allocate more bits to the individual ID field, typically, present-day trunked systems may recognize only approximately ten (10) thousand different individual IDs. Although originally sufficient, today, many trunked system operators are turning away new subscribers or have lists of subscribers waiting to be loaded.
Therefore, there is a need in the art to provide a trunked communication system that allows many thousand subscribers to be recognized (loaded on the system), while contemporaneously providing a signalling scheme designed to reduce control channel congestion, while still providing the advanced features that are now expected and required by trunked system users.